1. Field of the Invention
This invention relates to a method for making a semiconductor neuron that mimics the operation of a biological nerve cell and nerve fiber.
2. Background of the Invention
A biological nerve fiber in an animal structurally resembles a coaxial cable. The nerve fiber has an inner central core of an electrically conducting axoplasm surrounded by a thin membrane sheath and is immersed in the electrically conductive body fluids of the animal. All nerve cells and nerve fibers having a resting potential of 60-100 millivolts applied across the thin membrane sheath between the inner axoplasm core and surrounding body fluids. This resting potential is in a direction such that the inner axoplasm core is negatively charged with respect to the surrounding fluids and can be measured by penetrating the membrane sheath with a microelectrode and measuring the potential difference between the inner axoplasm core and the surrounding body fluids.
The membrane sheath of the nerve fiber normally acts as an insulating barrier while a nerve is resting. However, when the nerve is conducting nerve pulses, the membrane sheath actively responds to a depolarizing current (that is, a current that flows in a direction so as to eliminate the potential difference between the inner axoplasm core and the surrounding body fluids). For depolarizing currents above a critical threshold, typically 10.sup.-6 to 10.sup.-3 amps/cm.sup.2, the axon membrane actively increases the depolarizing current and adds to the original stimulus current. In other words, the membrane above a critical threshold depolarizing current acts as a differential negative resistor like a tunnel diode and actively assists depolarizing. In contrast, the membrane does not make any active response to a hyperpolarizing current (that is, a current that flows in a direction so as to increase the potential difference between the inner axoplasm core and the surrounding body fluids).
If a nerve is stimulated at one end by causing a depolarizing current greater than the critical threshold current to flow through the membrane sheath at the point of stimulus, the resistance of the membrane will decrease rapidly at this point allowing more current to pass through the membrane. This current will change the potential of the inner axoplasm core of the nerve at the point of stimulus. The change of potential will generate a flow of current in the axoplasm core away from the stimulus point along the inner core of the nerve fiber. As the current flows away from the stimulus point along the inner core, the current is gradually diverted through the surrounding membrane sheath and flows in a reverse direction through the surrounding body fluids back to the stimulus point. Here it again flows through the active membrane assisting the depolarizing current flow into the inner core and completing the current circuit. The area of the membrane actively boosting the depolarizing current spreads laterally along the axon as the depolarization current increases to the critical threshold in adjacent sections of the nerve fiber. At the same time at the original point of stimulus, the membrane which is acting like a capacitor discharging through a parallel differential negative resistance exhausts its ability to actively boost the depolarizing current which as a consequence decays at this point to zero. As a consequence of the differential negative resistance and the electrical capacitance of the membrane sheath of the nerve, a potential spike is formed which propagates away from the original stimulus point towards unstimulated sections of the nerve fiber. In this manner, a nerve pulse is conducted from one end of a nerve fiber to the other end.
An object of this invention is to provide a method for making a semiconductor neuron whose electrical properties will mimic those of a biological neuron.
Another object of this invention is to provide a method for making a tunnel diode by thermal gradient zone melting which will function as a semiconductor neuron.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.